Marine vessel operated with hybrid turbocharged internal combustion engine

ABSTRACT

A hybrid turbocharger includes a first power conversion unit to convert direct-current power into alternating-current power to be output to a generator motor, a smoothing capacitor between direct-current buses, and a control unit that controls the first power conversion unit so as to cause actual generator motor speed to comply with an engine speed command of the generator motor input from an upstream controller during a motoring operation of the generator motor. The control unit changes the engine speed command to a value which is equal to or greater than the actual generator motor speed if the engine speed command of the generator motor is less than the actual generator motor speed and a direct-current bus voltage is equal to or greater than a predetermined first threshold value during the motoring operation. Accordingly, the direct-current bus voltage can be prevented from increasing during the motoring operation.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/787,742, entitled “TURBOCHARGER AND MARINE VESSEL,” filed Oct. 28,2015, which claims priority to and is a 371 of International PatentApplication No.: PCT/JP2015/054878, filed Feb. 20, 2015 which isassigned to the assignee hereof and is expressly incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a turbocharger and a marine vesselincluding a turbocharger.

BACKGROUND

In the related art, there is a known hybrid turbocharger which not onlyutilizes engine exhaust gas discharged from an internal-combustionengine as a compressor driving force of the turbocharger but alsoutilizes the same as power for driving a generator motor, so as toobtain generating power. FIG. 10 illustrates a configuration of thehybrid turbocharger in the related art. As illustrated in FIG. 10, forexample, a hybrid turbocharger 100 includes an exhaust turbine 101 thatis driven by exhaust gas discharged from an internal-combustion enginesuch as a diesel engine for a marine vessel, a compressor 102 that isdriven by the exhaust turbine 101 and boosts outside air to theinternal-combustion engine, and a generator motor 103 that is joined torotary shafts of the exhaust turbine 101 and the compressor 102. Thegenerating power of an alternating-current obtained by the generatormotor 103 is converted into direct-current power by a converter 104.Thereafter, an inverter 105 inverts the direct-current power intothree-phase alternating-current power having a frequency correspondingto that of a system 106, thereby being supplied to the system 106. Asmoothing capacitor 107 for absorbing a fluctuation of a direct-currentvoltage is provided between the converter 104 and the inverter 105 whilebeing connected thereto.

Generally, in the hybrid turbocharger, a motoring operation (anoperation state in which electric power is supplied from an inverter toa generator motor) and a regenerating operation of the generator motor,are alternately determined by an upstream controller which controls theinternal-combustion engine. Therefore, the regenerating operation isprohibited when the motoring operation is selected by the upstreamcontroller. On the contrary, the motoring operation is prohibited whenthe regenerating operation is selected.

Patent Document 1: Japanese Unexamined Patent Application PublicationNo. 2012-214143

Patent Document 2: Japanese Unexamined Patent Application PublicationNo. 7-95775

SUMMARY

However, for example, if a motoring operation of a generator motor isselected by an upstream controller, if an engine speed command of thegenerator motor falls below actual generator motor speed, even though aregeneration operating is prohibited, a slight amount of motoring poweris generated and the motoring power is accumulated in a smoothingcapacitor, thereby resulting in the gradual increase of a direct-currentbus voltage. If the direct-current bus voltage increases and exceeds apredetermined value of tolerance, there is a possibility of theoccurrence of breakage, tripping, or the like in a semiconductor elementand a capacitor. The above-described problem occurs not only in a hybridturbocharger but also occurs in an apparatus including a compressorwhich is driven by a turbine and boosts outside air to aninternal-combustion engine, and an electric motor which is joined to arotary shaft of the compressor, in a similar manner, for example.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a schematic configuration of a marinevessel hybrid turbocharger in a first embodiment.

FIG. 2 is a schematic functional block diagram of a control unit in thefirst embodiment.

FIG. 3 is a flow chart illustrating an example of a procedure inswitching processing of an engine speed command which is executed by thecontrol unit in the first embodiment.

FIG. 4 is a diagram illustrating a relationship among an engine speed,the engine speed command, and a direct-current bus voltage in the firstembodiment.

FIG. 5 is a diagram for illustrating another example, that is, amodification example of the engine speed command in the firstembodiment.

FIG. 6 is a diagram illustrating a schematic configuration of thecontrol unit in a second embodiment.

FIG. 7 is a diagram illustrating a relationship among the engine speed,reactive power, and the direct-current bus voltage in the secondembodiment.

FIG. 8 is a diagram illustrating a schematic configuration of the marinevessel hybrid turbocharger in a third embodiment.

FIG. 9 is a diagram illustrating a relationship among the engine speed,the semiconductor switch, and the direct-current bus voltage in thethird embodiment.

FIG. 10 is a diagram illustrating a schematic configuration of themarine vessel hybrid turbocharger.

DETAILED DESCRIPTION

Particular embodiments may provide a turbocharger and a marine vesselhaving the turbocharger which can prevent the direct-current bus voltagefrom increasing during a motoring operation. According to a firstembodiment, there is provided a turbocharger including a compressor,which is driven by a turbine and boosts outside air to aninternal-combustion engine, and an electric motor which is joined to arotary shaft of the compressor. The turbocharger includes powerconversion unit that has a function by which direct-current power isconverted into alternating-current power and is output to the electricmotor, a smoothing capacitor that is provided in a direct-current buswhich is connected to the power conversion unit, and control unit thatcontrols the power conversion unit so as to cause actual generator motorspeed to comply with an engine speed command of the electric motor inputfrom an upstream controller during a motoring operation of the electricmotor. The control unit includes command change unit which changes theengine speed command to a value that approaches, such as a value that isequal to or greater than the actual generator motor speed, if the enginespeed command of the electric motor is less than the actual generatormotor speed and a line-to-line voltage of the smoothing capacitorapproaches, or is equal to or greater than, a predetermined firstthreshold value during the motoring operation of the electric motor.

According to the turbocharger, since the engine speed command is changedto the value equal to or greater than the actual generator motor speedif the engine speed command of the electric motor is less than theactual generator motor speed and the direct-current bus voltage is equalto or greater than, or at least approaches, the predetermined firstthreshold value during the motoring operation, electric power consumedin the electric motor can be increased. Accordingly, it is possible torelease an electrical charge accumulated in the smoothing capacitorwhich is provided between the direct-current buses. Thus, it is possibleto lower the direct-current bus voltage. The “actual generator motorspeed” of the electric motor is measured, for example, by an enginespeed measurement unit (not shown). For example, the engine speedmeasurement unit may comprise an engine speed sensor (not shown)provided in the electric motor or may comprise an engine speed which isestimated through computation performed based on a motor current and thelike. Regarding the motor current, the motor current applied from thepower conversion unit to the electric motor may be measured, orestimation may be performed based on a direct current and the likeflowing in the direct-current bus. The method of estimation performedbased on the direct current uses a current having noise components fewerthan those in the motor current. Thus, it is possible to enhanceaccuracy of the estimation.

In the turbocharger, the control unit may reset the engine speed commandto the engine speed command input from the upstream controller if theline-to-line voltage of the smoothing capacitor becomes equal to or lessthan a second threshold value which is a value equal to or less than thefirst threshold value from if the engine speed command is changed untilafter a predetermined period of time has elapsed.

In this case, since the engine speed command is reset to the originalengine speed command if the line-to-line voltage of the smoothingcapacitor, that is, the direct-current bus voltage becomes equal to orless than the second threshold value, it is possible to cause the enginespeed of the electric motor to comply with the engine speed commandwhich is applied from the upstream controller.

In the turbocharger, the command change unit may lower an increasingamount of the engine speed command if the engine speed command ischanged at a frequency equal to or greater than a predeterminedfrequency.

Power consumption can be slowed down by lowering the increasing amountof the engine speed command if the engine speed command is frequentlychanged. As a result, the rate of decline in the terminal voltage of thesmoothing capacitor can be slowed down. Thus, it is possible to preventthe engine speed command from being frequently switched.

In the marine vessel hybrid turbocharger, the command change unit mayset the engine speed command to a greater value if the line-to-linevoltage of the smoothing capacitor becomes no less than the secondthreshold value, even after a predetermined period of time has elapsedfrom if the engine speed command is changed.

The difference between the actual generator motor speed and the enginespeed command is increased by changing the engine speed command to thegreater value if there is an insignificant effect, even though theengine speed command is changed and the line-to-line voltage of thesmoothing capacitor does not become less than the second thresholdvalue, even after the predetermined period of time has elapsed.Accordingly, since electric power supplied to the electric motorincreases, the electrical charge of the smoothing capacitor can bereleased further. Thus, it is possible to lower the line-to-line voltageof the smoothing capacitor.

According to a second embodiment, there is provided a turbochargerincluding a compressor which is driven by a turbine and boosts outsideair to an internal-combustion engine, and an electric motor which isjoined to a rotary shaft of the compressor. The turbocharger includes apower conversion unit that has a function by which direct-current poweris converted into alternating-current power and is output to theelectric motor, a smoothing capacitor that is provided in adirect-current bus which is connected to the power conversion unit, andcontrol unit that applies a control signal to the power conversion unitso as to cause actual generator motor speed to comply with an enginespeed command of the electric motor input from an upstream controllerduring a motoring operation of the electric motor. The control unitgenerates a control command so as to increase an excitation currentcomponent and applies the control command to the power conversion unitif the engine speed command of the electric motor is less than theactual generator motor speed and a line-to-line voltage of the smoothingcapacitor is equal to or greater than a predetermined threshold valueduring the motoring operation of the electric motor.

According to the turbocharger, the excitation current component isincreased if the engine speed command of the electric motor is less thanthe actual, or measured, generator motor speed and the line-to-linevoltage of the smoothing capacitor, that is, the direct-current busvoltage is equal to or greater than the threshold value. Accordingly,reactive power consumed in the electric motor can be increased. As aresult, the electrical charge released from the smoothing capacitorprovided between the direct-current buses increases. Thus, it ispossible to lower the line-to-line voltage of the smoothing capacitor.

According to a third embodiment, there is provided a turbochargerincluding a compressor which is driven by a turbine and boosts outsideambient air to an internal-combustion engine, and an electric motorwhich is joined or coupled to a rotary shaft of the compressor. Theturbocharger includes a power conversion unit comprising a function bywhich direct-current power is converted into alternating-current powerand is output or sent to the electric motor, a smoothing capacitor thatis provided in a direct-current bus which is connected to the powerconversion unit, a resistor that is connected to the smoothing capacitorin parallel, a semiconductor switch that is connected to the resistor inseries and is in an open state while being in a steady state, andswitching control unit that controls ON-OFF of the semiconductor switch.The switching control unit performs ON-OFF ratio control of thesemiconductor switch within a predetermined duty cycle if the enginespeed command of the electric motor is less than the actual generatormotor speed and a line-to-line voltage of the smoothing capacitorapproaches, or is equal to or greater than, a predetermined thresholdvalue during a motoring operation of the electric motor.

According to the turbocharger, the ON-OFF ratio control of thesemiconductor switch is performed by the switching control unit if theengine speed command is less than the actual generator motor speed andthe line-to-line voltage of the smoothing capacitor, that is, thedirect-current bus voltage approaches, or becomes equal to or greaterthan, the threshold value during the motoring operation of the electricmotor. Accordingly, since the electrical charge of the smoothingcapacitor is consumed by resistance during the time period in which thesemiconductor switch is turned ON, the line-to-line voltage of thesmoothing capacitor can be lowered. Since the semiconductor switch isused as a switching element, the switching frequency can range fromseveral hundreds of Hz to several kHz. Thus, it is possible to preventovercurrent at the moment of discharging electricity.

In the turbocharger, the switching control unit may set the ON-OFF ratiowithin the duty cycle based on proportional-integral control to which adifference between the line-to-line voltage of the smoothing capacitorand the predetermined threshold value is input, and lower at least anyone of a proportional gain and an integral gain in theproportional-integral control by performing the ON-OFF ratio control ofthe semiconductor switch if a fluctuation of an engine speed of theelectric motor becomes equal to or greater than a predetermined value.

In this case, since at least any one of the proportional gain and theintegral gain in the proportional-integral control is lowered byperforming the ON-OFF ratio control of the semiconductor switch if thefluctuation of the engine speed of the electric motor becomes equal toor greater than the predetermined value, it is possible to slow down thedescending speed of the line-to-line voltage of the smoothing capacitor.

In the turbocharger, the switching control unit may increase at leastany one of the proportional gain and the integral gain in theproportional-integral control if the line-to-line voltage of thesmoothing capacitor does not become equal to or less than the thresholdvalue within a predetermined period of time from a start of the ON-OFFratio control of the semiconductor switch.

In this case, since at least any one of the proportional gain and theintegral gain in the proportional-integral control is increased if theline-to-line voltage of the smoothing capacitor does not become equal toor less than the threshold value within the predetermined period of timefrom the start of the ON-OFF ratio control of the semiconductor switch,it is possible to quicken the descending speed of the line-to-linevoltage of the smoothing capacitor.

According to a fourth embodiment, there is provided a marine vesselwhich includes the above-described turbocharger and aninternal-combustion engine to which the turbocharger boosts outside air.

According to a embodiment, there is provided a method in which asmoothing capacitor prevents a fluctuation of a voltage ofdirect-current power, and the direct-current power is converted intoalternating-current power and is output or sent to an electric motor soas to bring about actual or measured generator motor speed to complywith an engine speed command of the electric motor. The engine speedcommand is changed to a value which is greater than the actual generatormotor speed, and the direct-current power is converted into thealternating-current power and is output to the electric motor based onthe changed engine speed command if the engine speed command of theelectric motor is less than the actual generator motor speed and aline-to-line voltage of the smoothing capacitor is equal to or greaterthan a predetermined threshold value during a motoring operation of theelectric motor.

According to a sixth embodiment, there is provided a method in which asmoothing capacitor prevents a fluctuation of a voltage ofdirect-current power, and the direct-current power is converted intoalternating-current power and is output to an electric motor so as tobring about a measured generator motor speed to comply with an enginespeed command of the electric motor. A control command is generated soas to increase an excitation current component, and the direct-currentpower is converted into the alternating-current power and is output tothe electric motor if the engine speed command of the electric motor isless than the actual generator motor speed and a line-to-line voltage ofthe smoothing capacitor is equal to or greater than a predeterminedthreshold value during a motoring operation of the electric motor.

According to a second embodiment, there is provided a method in which asmoothing capacitor prevents a fluctuation of a voltage ofdirect-current power, and the direct-current power is converted intoalternating-current power and is output or sent to an electric motor soas to bring about actual or measured generator motor speed to complywith an engine speed command of the electric motor. The resistor isconnected to the smoothing capacitor in parallel, and then, asemiconductor switch, which is in an open state while being in a steadystate, is connected to the resistor in series. ON-OFF ratio control ofthe semiconductor switch is performed within a predetermined duty cycleif the engine speed command of the electric motor is less than theactual generator motor speed and a line-to-line voltage of the smoothingcapacitor is equal to or greater than a predetermined threshold valueduring a motoring operation of the electric motor.

Accordingly, embodiments provide an effect of being able to prevent adirect-current bus voltage from increasing during a motoring operation.

Hereinafter, embodiments in a case in which a turbocharger is applied toa marine vessel as a hybrid turbocharger will be described withreference to the drawings.

FIG. 1 is a diagram illustrating a schematic configuration of the marinevessel hybrid turbocharger (hereinafter, referred to as “the hybridturbocharger”) in a first embodiment. As illustrated in FIG. 1, a hybridturbocharger 10 includes an exhaust turbine 21 that is driven by exhaustgas discharged from a diesel engine (an internal-combustion engine (notshown)) for a marine vessel, a compressor 23 that is driven by theexhaust turbine 21 and boosts outside air to the internal-combustionengine, and a generator motor 30 that is joined or coupled to rotaryshafts of the exhaust turbine 21 and the compressor 23. The hybridturbocharger 10 not only utilizes the engine exhaust gas discharged fromthe diesel engine for a marine vessel as a compressor driving force ofthe turbocharger but also utilizes the same as power for driving thegenerator motor 30, so as to obtain generating power.

The hybrid turbocharger 10 includes a power conversion apparatus 20which is provided between the generator motor 30 and an onboardelectrical grid 16. The power conversion apparatus 20 includes a firstpower conversion unit 12, a second power conversion unit 14, and asmoothing capacitor 18 as the main configuration.

The first power conversion unit 12 causes, or brings about, thegenerating power of the generator motor 30 to be converted intodirect-current power and to be output during the regenerating operationof the generator motor 30, and the first power conversion unit 12causes, or brings about, the direct-current power to be converted intoalternating-current power and to be output to the generator motor 30during the motoring operation. The second power conversion unit 14causes, or brings about, the direct-current power from the first powerconversion unit 12 to be converted into three-phase alternating-currentpower suitable for the system and to be output to the onboard electricalgrid 16 during the regenerating operation of the generator motor 30, andthe second power conversion unit 14 causes, or brings about, thethree-phase alternating-current power from the onboard electrical grid16 to be converted into the direct-current power during the motoringoperation and to be output or sent to the first power conversion unit12. The smoothing capacitor 18 is provided between direct-current busesbetween the first power conversion unit 12 and the second powerconversion unit 14, thereby reducing a fluctuation of a direct-currentvoltage. The actual generator motor speed of the generator motor 30 ismeasured by general engine speed measurements or is determined based ona voltage fluctuation cycle. Actual generator motor speed N istransmitted to a control unit 40.

The first power conversion unit 12 and the second power conversion unit14 described above is configured to comprise a circuit in which sixswitching elements are subjected to bridge connection. Here, theconfigurations of the first power conversion unit 12 and the secondpower conversion unit 14 are not limited to the above-describedexamples, and other configurations can be employed. For example, aconfiguration including a rectifier circuit in which six diode elementsare subjected to bridge connection and a circuit in which thedirect-current voltage generated by the rectifier circuit is stepped upand down by using one or two switching elements may be employed.Otherwise, a configuration in which “circuits configured to have the sixswitching elements which are subjected to bridge connection” or“circuits in which the direct-current voltage generated by the rectifiercircuit in which the six diode elements are subjected to bridgeconnection are stepped up and down by using one or two switchingelements” (so called multi-level circuits) are combined in multi levelsmay be employed. In addition, a matrix conversion configurationconfigured to have nine switching elements may be employed. Furthermore,a configuration in which the matrix conversion configurations are formedin multi levels may be employed.

The first power conversion unit 12 is controlled by the control unit 40.There is also provided a control unit (not illustrated) for controllingthe second power conversion unit 14, but the description thereof will beomitted herein. In the hybrid turbocharger 10, a voltage detection unit17 for detecting a direct-current bus voltage, that is, a line-to-linevoltage of the smoothing capacitor 18 is provided between thedirect-current buses.

FIG. 2 is a schematic functional block diagram of the control unit 40.The control unit 40 has a function to control the first power conversionunit 12 so as to cause, or brings about, the engine speed of thegenerator motor 30 to coincide with an engine speed command N* duringthe motoring operation of the generator motor 30. Whether to perform themotoring operation or the regenerating operation for the generator motor30 is determined based on a command from an upstream controller 50. Inother words, if making selection between the motoring operation and theregenerating operation, a relationship between the engine speed commandN* and the actual generator motor speed N of the generator motor 30 isnot considered. Therefore, if the motoring operation is selected in theupstream controller 50, the apparatus is controlled so as not to performthe regenerating operation during the motoring operation, even thoughthe actual generator motor speed N exceeds the engine speed command N*.

As illustrated in FIG. 2, the control unit 40 includes an engine speedcommand generation unit 41, an engine speed command switching unit 42, adifference computation unit 43, and a control signal generation unit 44.The present embodiment illustrates a case where engine speed commandchange unit is realized by the engine speed command generation unit 41and the engine speed command switching unit 42. The actual generatormotor speed N of the generator motor 30 is input to the engine speedcommand generation unit 41, so as to generate an engine speed commandNb* which is obtained by increasing the actual generator motor speed Nby a predetermined amount. An engine speed command Na* from the upstreamcontroller 50, the actual generator motor speed N of the generator motor30, the engine speed command Nb* from the engine speed commandgeneration unit 41, and a direct-current bus voltage Vdc from thevoltage detection unit 17 are input to the engine speed commandswitching unit 42. The engine speed command switching unit 42 outputsthe engine speed command Na* from the upstream controller 50 as theengine speed command N* if the direct-current bus voltage Vdc is lessthan a predetermined first threshold value and outputs the engine speedcommand Nb* from the engine speed command generation unit 41 as theengine speed command N* if the direct-current bus voltage Vdc is equalto or greater than the first threshold value during the motoringoperation of the generator motor 30.

The difference computation unit 43 calculates a difference ΔN betweenthe engine speed command N* output from the engine speed commandswitching unit 42 and the actual generator motor speed N of thegenerator motor 30 and outputs the difference ΔN to the control signalgeneration unit 44. The control signal generation unit 44 performsproportional-integral control and the like with respect to thedifference between ΔN from the difference computation unit 43, therebygenerating a control signal S of the first power conversion unit 12 forcausing, or bringing about, the actual generator motor speed N tocoincide with the engine speed command N*. For example, the controlsignal generation unit 44 generates a PWM signal for controlling ON-OFFof the switching elements included in the first power conversion unit12. Since there are a number of known technologies regarding a controlmethod of generating the PWM signal which causes the actual generatormotor speed N to coincide with the engine speed command N*, knowntechnologies may be suitably employed. If there is no sensor, the actualgenerator motor speed N may be an estimated value.

Subsequently, among the methods of processing executed by the controlunit 40, switching processing of the engine speed command will be mainlydescribed with reference to FIG. 3. FIG. 3 is a flow chart illustratingan example of the procedure in switching processing of the engine speedcommand executed by the control unit 40.

First, if the direct-current bus voltage Vdc is input during themotoring operation (Step SA1), it is determined whether or not thedirect-current bus voltage Vdc is equal to or greater than the firstthreshold value (Step SA2). If the direct-current bus voltage Vdc isless than the first threshold value, the procedure returns to Step SA1,and if the direct-current bus voltage Vdc is equal to or greater thanthe first threshold value, or approaches the first threshold value, theengine speed command Nb* is computed by multiplying the actual generatormotor speed N by a coefficient α (for example, α=1.01) (Step SA3). Here,the coefficient α is set based on the coefficient table which is set inadvance, and the initial value is set to have α=1.01.

Subsequently, the engine speed command N* is switched from the enginespeed command Na* which is input from the upstream controller 50 to theengine speed command Nb* which is computed in Step SA3 (Step SA4). Insuccession, based on a relationship between required torque and inertiaof the generator motor 30, a time T taken until the actual generatormotor speed N of the generator motor 30 coincides with the engine speedcommand Nb* calculated in Step SA3 is computed (Step SA5).

Subsequently, it is determined whether or not the actual generator motorspeed N reaches the engine speed command Nb* before the direct-currentbus voltage Vdc becomes less than a second threshold value (the firstthreshold value≥the second threshold value) (Step SA6). If the conditionis satisfied (“YES” in Step SA6), the engine speed command N* isswitched to the engine speed command Na* input from the upstreamcontroller 50 (Step SA7). In succession, setting is performed to havethe coefficient α=1.01 (Step SA8), and the procedure returns to StepSA1. Meanwhile, in Step SA6, if the above-described condition is notsatisfied (“NO” in Step SA6), it is determined whether or not thedirect-current bus voltage Vdc becomes less than the second thresholdvalue within the T time after the engine speed command N* is switched toNb* (Step SA9). As a result, if the direct-current bus voltage Vdc doesnot become less than the second threshold value within the T time (“NO”in Step SA9), the procedure proceeds to Step SA7. Meanwhile, if “YES” inStep SA9, the engine speed command N* is switched to the engine speedcommand Na* input from the upstream controller 50 (Step SA10).Subsequently, within a predetermined time period from the time if thedirect-current bus voltage Vdc becomes equal to or greater than thefirst threshold value, it is determined whether or not switching of theengine speed command N* is performed equal to or more than apredetermined number of times (Step SA11). For example, thepredetermined time period may be the same as the time T which iscalculated in Step SA5 as described above and may be set as a timeperiod longer than the time T. In Step SA11, if switching of the enginespeed command N* is performed a number of times that is equal to or morethan the predetermined times within the predetermined time period,setting is performed to have the coefficient α=1.009 (Step SA12), andthe procedure returns to Step SA1. Meanwhile, in Step SA11, if switchingof the engine speed command N* is performed a number of times that isless than the predetermined times within the predetermined time period,the processing ends.

As described above, according to the hybrid turbocharger 10 and a methodof controlling the same in the present embodiment, if the engine speedcommand Na* from the upstream controller 50 is less than the actualgenerator motor speed N of the generator motor 30 and the direct-currentbus voltage Vdc is equal to or greater than the predetermined firstthreshold value during the motoring operation, the engine speed commandN* is changed to a value greater than the actual generator motor speedN, for example, to the engine speed command Nb* which is obtained bymultiplying the actual generator motor speed N by the predeterminedcoefficient α (however, α>1), and the first power conversion unit 12 iscontrolled so as to cause, or bring about, a measured or actualgenerator motor speed N to coincide with the changed engine speedcommand N*. Accordingly, the electrical charge accumulated in thesmoothing capacitor 18 provided between the direct-current buses can bereleased, and thus, the direct-current bus voltage Vdc can be lowered.

FIG. 4 illustrates a relationship among the engine speed, the enginespeed command, and the direct-current bus voltage in the presentembodiment. At the time t1 in FIG. 4, if the engine speed command fallsbelow the actual generator motor speed, the direct-current bus voltageincreases gradually. At the time t2, if the direct-current bus voltageexceeds the first threshold value, electric power supplied to thegenerator motor 30 increases by setting the engine speed command to theengine speed command Nb* which is greater than the actual generatormotor speed, and the electrical charge of the smoothing capacitor 18 isreleased. Thus, the direct-current bus voltage is lowered gradually. Ifthe direct-current bus voltage is lowered to the second threshold value(for example, a steady voltage), the engine speed command N* is switchedto the engine speed command Na* from the upstream controller 50. Inaccordance therewith, the direct-current bus voltage rises gradually.

According to the present embodiment, if switching of the engine speedcommand N* described above is frequently performed due to thedirect-current bus voltage Vdc which frequently exceeds the thresholdvalue, the coefficient α, by which the actual generator motor speed N ismultiplied may be sequentially changed to smaller values in stages setin advance (for example, 1.01, 1.009, and 1.0008). Accordingly, the rateof decline of the direct-current bus voltage Vdc can be slowed down, andthus, the engine speed command N* can be prevented from being frequentlyswitched.

On the contrary, if the direct-current bus voltage Vdc does not becomeequal to or less than the threshold value, or does not approach thethreshold value, even after the predetermined time period T has elapsed,for example, the value of the coefficient α by which the actualgenerator motor speed N is multiplied may be changed to greater valuesin stages. In this manner, if there is an insignificant effect withoutsuccess in lowering the direct-current bus voltage Vdc, even though theengine speed command N* is changed, the value of the coefficient α issequentially set to greater values in stages set in advance (forexample, 1.01, 1.011, and 1.012), thereby increasing the differencebetween the engine speed command N* and the actual generator motor speedN. Accordingly, power supplied to the generator motor 30 increases.Therefore, the electrical charge of the smoothing capacitor 18 can bereleased further, and the direct-current bus voltage Vdc can be lowered.

In the present embodiment, if the engine speed command Na* from theupstream controller 50 becomes less than the actual generator motorspeed N and the direct-current bus voltage Vdc is equal to or greaterthan the first threshold value, the engine speed command N* is changedto the engine speed command Nb*, which is a value greater than theactual generator motor speed. However, for example, in place thereof, asillustrated in FIG. 5, the engine speed command N* may be set so as tocause the engine speed command N* to coincide with the actual generatormotor speed N during a period of time in which the engine speed commandN* applied from the upstream controller 50 is smaller than the actualgenerator motor speed N. As the engine speed command N* set in thismanner, without changing the control on the onboard electrical grid 16side, a current can be prevented from flowing from the generator motor30 to the onboard electrical grid 16 side by using the power conversionapparatus 20 and the control unit 40 thereof provided on the hybridturbocharger 10 side. Thus, the direct-current bus voltage Vdc can beprevented from rising and can be uniformly maintained at the steadyvalue.

Subsequently, the hybrid turbocharger and the method of controlling thesame according to a second embodiment will be described with referenceto the drawings. In the hybrid turbocharger and the method ofcontrolling the same according to the first embodiment described above,the engine speed command N*, which is greater than the actual generatormotor speed N is set so as to lower the direct-current bus voltage Vdcif the direct-current bus voltage Vdc exceeds the first threshold valueduring the motoring operation. However, in the present embodiment,consumption of reactive power in the generator motor 30 is increased byincreasing an excitation current component command which is referred ifgenerating the control signal output to the first power conversion unit12, and the direct-current bus voltage Vdc is lowered by increasing lossof the generator motor. Hereinafter, the hybrid turbocharger and themethod thereof according to the present embodiment will be describedmainly regarding the points different from those in the first embodimentdescribed above.

FIG. 6 is a diagram illustrating a schematic configuration of a controlunit 40′ in the present embodiment. As illustrated in FIG. 6, thecontrol unit 40′ includes a three-phase/two-phase conversion unit 51, arotation/fixation conversion unit 52, an engine speed PI control unit53, an excitation current component command generation unit 54, acurrent PI control unit 56, a fixation/rotation conversion unit 57, atwo-phase/three-phase unit 58, and a PWM signal generation unit 59, anda magnetic flux estimation unit 60.

The three-phase/two-phase conversion unit 51 converts a three-phasealternating current flowing in the generator motor 30 into a two-phasecurrent (α-phase and β-phase). For example, conversion is performedwhile using a current of one phase among the currents of three phases asa reference. In FIG. 6, a current Iu of a u-phase and a current Iv of av-phase are input to the three-phase/two-phase conversion unit 51.However, a current Iw of a W-phase is calculated by performingcomputation based on the currents Iu and Iv of the two phases thereof.The rotation/fixation conversion unit 52 performs coordinatetransformation with respect to currents Iα and Iβ subjected to two-phaseconversion by the three-phase/two-phase conversion unit 51 while usingrotor positions as a reference, thereby obtaining a d-axis current Idand a q-axis current Iq.

The engine speed PI control unit 53 calculates a q-axis current command(a torque current command) Iq* by executing the proportional-integralcontrol with respect to the difference between the engine speed commandN* and the engine speed N of the generator motor 30 so as to cause, orbring about, the actual generator motor speed N of the generator motor30 (here, an engine speed estimated by the below-described magnetic fluxestimation unit 60 is used as the engine speed N) to coincide with theengine speed command N* input from the upstream controller 50.

The excitation current component command generation unit 54 sets anexcitation current component command Id* to 0 (zero) during the motoringoperation if the direct-current bus voltage Vdc is less than thepredetermined first threshold value, and sets the excitation currentcomponent command Id* to α (α>0) during the motoring operation if thedirect-current bus voltage Vdc is equal to or greater than thepredetermined first threshold value.

The current PI control unit 56 generates a q-axis voltage command Vq* byexecuting the proportional-integral control with respect to thedifference between the q-axis current Iq obtained by therotation/fixation conversion unit 52 and the q-axis current command Iq*output from the engine speed PI control unit 53. Similarly, a d-axisvoltage command Vd* is generated by executing the proportional-integralcontrol with respect to the difference between the d-axis current Idobtained by the rotation/fixation conversion unit 52 and the d-axiscurrent command Id* output from the excitation current component commandgeneration unit 54. The fixation/rotation conversion unit 57 obtains anα-phase voltage command Vα* and a β-phase voltage command Vβ* byperforming reverse coordinate transformation with respect to the q-axisvoltage command Vq* and the d-axis voltage command Vd* obtained by thecurrent PI control unit 56 while using the rotor positions as areference. The two-phase/three-phase unit 58 obtains three-phase voltagecommands Vu*, Vv*, and Vw* by performing transformation reverse to thecoordinate transformation performed by the three-phase/two-phaseconversion unit 51, with respect to the α-phase voltage command Vα* andthe β-phase voltage command Vβ*. The PWM signal generation unit 59generates a PWM signal S by comparing the three-phase voltage commandsVu*, Vv*, and Vw* and a carrier signal.

The magnetic flux estimation unit 60 estimates the magnetic fluxgenerated in a rotor based on the two-phase currents Iα and Iβ circuitconstants (inductance, resistance, and the like), and the two-phasevoltages Vα and Vβ, and a variable frequency obtained by multiplyingrotor angles by sine and cosine is estimated based on the magnetic fluxgenerated in a rotor. Since the variable frequency becomes a multipliedvalue of the counter electrode of the rotation frequency of the rotor, arotor angle θ can be estimated by utilizing a phase-locked loop.

According to such a control unit 40′, first, two phases (U-phase andV-phase) among the three-phase currents (U-phase, V-phase, and W-phase)flowing in the generator motor 30 are detected, and the two-phasecurrents are input to the three-phase/two-phase conversion unit 51. Thethree-phase/two-phase conversion unit 51 calculates the remainingone-phase current Iw based on the input two-phase currents Iu and Iv,and the three-phase currents Iu, Iv, and Iw are converted into thetwo-phase currents Iα and Iβ. In succession, the rotation/fixationconversion unit 52 converts the two-phase currents Iα and Iβ into thed-axis current Id and the q-axis current Iq.

The engine speed PI control unit 53 calculates the difference betweenthe engine speed N of the generator motor 30 and the engine speedcommand N* applied from the upstream controller 50, and PI control isexecuted with respect to the difference therebetween, therebycalculating the q-axis current command Iq*. The excitation currentcomponent command generation unit 54 performs setting of Id*=0 if thedirect-current bus voltage Vdc is less than the predetermined firstthreshold value, and performs setting of Id*=α (α>0) if thedirect-current bus voltage Vdc is equal to or greater than thepredetermined first threshold value. In this case, the value of thed-axis current command (the excitation current component command) may beset in accordance with the value of the direct-current bus voltage Vdc.For example, information (table, function, and the like) in which thedirect-current bus voltage Vdc and the d-axis current command areassociated with each other may be retained so as to set the d-axiscurrent command corresponding to the direct-current bus voltage Vdc, byusing the information. In this case, the d-axis current command Id* isset to a greater value as the direct-current bus voltage Vdc becomesgreater.

The current PI control unit 56 calculates the q-axis voltage command Vq*and the d-axis voltage command Vd* for causing the q-axis current Iq andthe d-axis current Id to respectively coincide with the q-axis currentcommand Iq* and the d-axis current command Id*, and as the q-axisvoltage command Vq* and the d-axis voltage command Vd* pass through thefixation/rotation conversion unit 57 and the two-phase/three-phaseconversion unit 58, the three-phase voltage commands Vu*, Vv*, and Vw*are calculated. Then, the PWM signal generation unit 59 generates thePWM signal S based on the three-phase voltage commands Vu*, Vv*, andVw*. The PWM signal S is applied to the first power conversion unit 12so as to control ON-OFF of the six switching elements included in thefirst power conversion unit 12.

As described above, according to the hybrid turbocharger and the methodof controlling the same in the present embodiment, controlling similarto that during normal usage is performed if the direct-current busvoltage is less than the predetermined first threshold value during themotoring operation of the generator motor 30, but the d-axis currentcommand is corrected to a value greater than that during normal usage ifthe direct-current bus voltage is equal to or greater than the firstthreshold value. Accordingly, the reactive power consumed in thegenerator motor 30 can be increased. As a result, the releasedelectrical charge of the smoothing capacitor 18 provided between thedirect-current buses increases. Thus, the direct-current bus voltage Vdccan be lowered.

FIG. 7 illustrates a relationship among the engine speed, reactivepower, and the direct-current bus voltage in the present embodiment. InFIG. 7, the direct-current bus voltage increases gradually if the enginespeed command falls below the actual generator motor speed at the timet1. Then, since the d-axis current command increases if thedirect-current bus voltage exceeds the first threshold value at the timet2, the reactive power increases. In addition, in accordance therewith,the direct-current bus voltage is lowered gradually. Then, since thecorrection value of the d-axis current command is reset to zero if thedirect-current bus voltage is lowered to the second threshold value (forexample, the steady voltage), the reactive power becomes zero. Since thed-axis current command (the excitation current component command) doesnot contribute to the engine speed of the generator motor 30, the enginespeed of the generator motor is not changed, even though the d-axiscurrent command is changed. Accordingly, it is possible to maintainstable control of the engine speed.

Subsequently, the hybrid turbocharger and the method of controlling thesame according to a third embodiment will be described with reference tothe drawings. In the hybrid turbocharger and the method of controllingthe same according to the first embodiment and the second embodimentdescribed above, the direct-current bus voltage Vdc is lowered bychanging the control of the first power conversion unit 12 performed bythe control units 40 and 40′. However, the present embodiment isdifferent from the first embodiment and the second embodiment describedabove in that a hardware configuration is newly provided in order tolower the direct-current bus voltage Vdc. Hereinafter, the hybridturbocharger and the method thereof according to the present embodimentwill be described mainly regarding the points different from those inthe first embodiment described above.

FIG. 8 is a diagram illustrating a schematic configuration of the hybridturbocharger in the present embodiment. The same reference numerals andsigns are applied to the configurations common to those in FIG. 1, andthe descriptions thereof will be omitted. As illustrated in FIG. 8, ahybrid turbocharger 10′ according to the present embodiment additionallyincludes a resistor 70 which is connected to the smoothing capacitor 18in parallel, a semiconductor switch 71 which is connected to theresistor 70 in series, and a switching control unit 72 for controllingON-OFF of the semiconductor switch 71. The semiconductor switch 71 is inan open state while being in the steady state, and an insulated gatebipolar transistor (IGBT), a field effect transistor (FET), and the likecan be exemplified.

The switching control unit 72 performs ON-OFF ratio control of thesemiconductor switch 71 within a predetermined duty cycle if the enginespeed command of the generator motor 30 is less than the actualgenerator motor speed and the direct-current bus voltage Vdc is equal toor greater than the predetermined first threshold value during themotoring operation of the generator motor 30.

For example, the switching control unit 72 determines the switching dutyby executing the proportional-integral control with respect to thedifference between the direct-current bus voltage Vdc and the thresholdvalue. As the ON-OFF ratio control of the semiconductor switch 71 isperformed, the switching control unit 72 lowers at least any one of aproportional gain and an integral gain in the proportional-integralcontrol if the fluctuation of the engine speed N of the generator motor30 becomes equal to or greater than the predetermined value.Accordingly, the descending speed of the direct-current bus voltage canbe slowed down.

The switching control unit 72 increases at least any one of theproportional gain and the integral gain in the proportional-integralcontrol if the direct-current bus voltage Vdc does not become equal toor less than the second threshold value (for example, the steadyvoltage) within the predetermined period of time from the start of theON-OFF ratio control of the semiconductor switch 71. Accordingly, thedescending speed of the direct-current bus voltage Vdc can be quickened.

In such a hybrid turbocharger 10′, the ON-OFF ratio control of thesemiconductor switch 71 is performed by the switching control unit 72 ifthe engine speed command input from the upstream controller becomes lessthan the actual generator motor speed and the direct-current bus voltageVdc becomes equal to or greater than the threshold value during themotoring operation of the generator motor 30. Accordingly, theelectrical charge of the smoothing capacitor 18 is consumed by theresistor 70 during the period of time in which the semiconductor switch71 is turned ON, and thus, the direct-current bus voltage Vdc can belowered. Since the semiconductor switch 71 is used as the switchingelement, the switching frequency can range from several hundreds of Hzto several kHz. Thus, it is possible to prevent overcurrent at themoment of discharging electricity.

FIG. 9 illustrates a relationship among the engine speed, the ON-OFFratio control of the semiconductor switch, and the direct-current busvoltage in the present embodiment. The direct-current bus voltagegradually increases if the engine speed command falls below the actualgenerator motor speed at the time t1 in FIG. 9. Then, since thesemiconductor switch starts to be turned ON and OFF at the time t2 ifthe direct-current bus voltage exceeds the first threshold value, theelectrical charge of the smoothing capacitor 18 is discharged by theresistor 70. Accordingly, the direct-current bus voltage is loweredgradually. Then, the semiconductor switch stops being turned ON and OFFif the direct-current bus voltage is lowered to the voltage value duringnormal usage.

Claimed subject matter is not limited to only the embodiments describedabove, and various modifications can be executed by combining each ofthe above-described embodiments partially or in the entirety thereofwithout departing from the gist and the scope of claimed subject matter,for example. For example, in the embodiments described above, an exampleis described regarding the case where the turbocharger of claimedsubject matter is applied to a marine vessel as the marine vessel hybridturbocharger. However, the turbocharger of claimed subject matter can beapplied not only to a marine vessel but can also be applied to otherapparatuses. In the embodiments described above, the example isdescribed regarding the case where the generator motor 30 is included asthe electric motor which can perform both the regeneration (powergeneration) operation and the motoring operation. However, an electricmotor which can perform only the motoring operation without having thefunction of regeneration may be employed in place of the generator motor30. In this case, an inverter in which the direct-current power isconverted into the alternating-current power and is output may beemployed as the power conversion unit.

The invention claimed is:
 1. A method of preventing a fluctuation of avoltage of direct-current power, and the direct current power beingconverted into alternating-current power in a power conversion unit of aturbocharged internal combustion engine including a turbocharger havinga compressor driven by a turbine via a shaft, an electric motor coupledto the compressor of the turbocharger, a smoothing capacitor coupled tothe power conversion unit, and a control unit providing a controlsignal, the method comprising; generating an actual generator motorspeed to comply with a generator speed command of the electric motorwhile monitoring an operation of the electric motor by the control unit;and increasing an excitation current value by the control unit if thegenerator motor speed command of the electric motor is less than theactual generator motor speed and a line-to-line voltage of the smoothingcapacitor approaches a predetermined threshold value that was set forthe line-to-line voltage during operation of the electric motor.
 2. Aturbocharger comprising: a compressor driven by a turbine to boostambient air to an internal combustion engine; an electric motor coupledto a rotary shaft of the compressor; a power conversion unit to convertdirect-current power to alternating-current power to supply the electricmotor; a smoothing capacitor coupled to a direct-current bus, thedirect-current bus coupled to the power conversion unit; and a controlunit to send a control signal to the power conversion unit to reach ameasured value of a generator motor speed to comply with a value of agenerator motor speed command of an input signal to the electric motor,from an upstream controller, during operation of the electric motor,wherein the control unit to generate a control command to increase anexcitation current value and to send the control command to the powerconversion unit if the value of the generator motor speed command of theelectric motor is less than a value of the measured generator motorspeed and a line-to-line voltage of the smoothing capacitor is equal toor greater than a predetermined threshold value that was set for theline-to-line voltage during operation of the electric motor.
 3. A marinevessel comprising: a turbocharger, including: a compressor to be drivenby a turbine of the marine vessel to boost ambient air to an internalcombustion engine; an electric motor coupled to a rotary shaft of thecompressor; a power conversion unit to convert direct-current power toalternating-current power to supply the electric motor; a smoothingcapacitor coupled to a direct-current bus, the direct-current buscoupled to the power conversion unit; a control unit to send a controlsignal to the power conversion unit to reach a measured value of agenerator motor speed to comply with a value of a generator motor speedcommand of an input signal to the electric motor, from an upstreamcontroller, during operation of the electric motor, wherein the controlunit is to generate a control command to increase a value of anexcitation current value and to send the control command to the powerconversion unit if the value of the generator motor speed command of theelectric motor is less than a measured value of the generator motorspeed and a line-to-line voltage of the smoothing capacitor is equal toor greater than a predetermined threshold value that was set for theline-to-line voltage during operation of the electric motor.